The near 24-h oscillations in neural, hormonal and metabolic activities, present in all mammals, are controlled by a biological clock that is intrinsic to the suprachiasmatic nuclei (SCN) of the ventral hypothalamus. Light is a primary signal responsible for resetting the clock. Light phase delays the clock in the early night, phase advances the clock in the late night, but has no effect during the day. I have previously demonstrated the cGMP and protein kinase G(PKG) are essential elements of the signal transduction pathway utilized by light to phase advance the clock in the latter half of the night. In fact, the last half of the night appears to be dominated by cGMP mediated events. cGMP levels and PKG activity rise spontaneously just before dawn. Inhibition of PKG at this time forces to repeat the last 3 h of its cycle. Thus, cGMP/PKG may regulate a critical clock control point at the end of the end of the night; without the endogenous oscillation of cGMP/PKG, the clock cannot progress from night into day. Light also causes phase advances of circadian rhythms in Drosophila. The mechanism is through break down of the protein product of an essential clock gene, timeless(TIM). Because TIM levels and phosphorylation state increase toward the end of the night, and TIM is degraded by light, it is likely that TIM may be a target for phosphorylation at the end of the night in the SCN. This proposal seeks to determine if regulation of a TIM-like protein in the mammalian SCN is a part of the critical mechanism, regulated by PKG at the end of the night. These studies will provide the first evidence of a mammalian TIM-like protein, as well as provide a direct functional link between cellular, PKG-medicated events and molecular elements of the clock.